Star Gemstones, Cats Eyes Quality and Causes

Of special interest in the study of
gemology is the subject of phenomena.
Briefly, as applied here, it is described as those unusual optical effects
displayed by some gem varieties. The interesting crossed lines of reflected
light seen in star sapphires and star rubies, the billowy bluish sheen
displayed by moonstone, and the vivid multiple flashes of color observed
in fine opals are all examples of phenomena. These and other unusual
effects will be studied in this assignment.

The phenomenal effects observed in gems may be classified under one
of three main types:

Reflection of light from inclusions,

Selective absorption of light, and

Interference of light.

Star Phenomena Resulting From Reflection of Light

Chatoyancy (pronounced shah-TOY-an-see)

Chatoyancy is the property of a gemstone to reflect light in a line
resembling the contracted pupil of a cat's eye. In a fine cat's-eye
it appears as a narrow, well-defined light-colored streak across the
summit of the stone that moves as the stone is turned. Cat's-eye is
a variety of the gem species chrysoberyl. When chrysoberyl exhibits
a movable light that is not confined to a narrow band, it is usually
called CYMOPHANE (pronounced CY-mo-fane), although the term is sometimes
applied to chrysoberyl that exhibits a movable light effect of any kind.

Fine cat's-eye contains a vast number of minute, parallel needlelike
inclusions that may be cavities or, more often, included fibrous crystals.
The light-colored streak, which is always at right angles to the fibers,
is produced by reflections from these inclusions (Figure 1). A high
or medium-high cabochon results in the most desirable "eye," since this
form of cutting limits the reflections that are visible from anyone
direction to a sharp band of light. In a flat-topped stone, reflections
are seen from the entire surface (sheer). In a cabochon cut, the light
reflected from the Inclusions is refracted out at different angles;
thus only a narrow band of light is seen from anyone direction (Figure
2).

A form of chatoyancy may be observed by holding a spool of glossy
thread under a single light source. A narrow chatoyant band will be
visible on the apex of the spool at right angles to the threads. The
round form of the spool, serves the same function as the cabochon cut
of a chatoyant gem.

Tiger's-eye, a variety of quartz often incorrectly called "crocidolite",
is usually a yellow-brown stone with a wavy fibrous structure. The appearance
is similar to a series of broad cat's-eye effects. It may be dyed any
number of colors. Tiger's-eye was formed when crocidolite, or blue asbestos,
a fibrous mineral, was replaced by quartz but whose fibrous structure
was preserved. In other words, the quartz is a pseudomorph after crocidolite.
When cut in cabochon, a cat's-eye effect is produced by reflection from
these fibers. The blue variety, which probably contains unaltered crocidolite,
is sometimes called hawk's-eye. There is also a fine cat's-eye variety
of quartz that resembles its more valuable counterpart, chrysoberyl
cat's-eye. This type has much finer inclusions than tiger's-eye, but
lacks the silky transparency of chrysoberyl. There are a number of other
gemstones in which cat's-eye varieties are more or less uncommon. These
include tourmaline, beryl, feldspar and a few infrequently encountered
gemstones such as scapolite, diopside and enstatite.

Judgment of quality of a cat's-eye is based on color, the position
of the eye (it should be straight and thin and follow the crest of the
cabochon exactly when the light source is directly over the stone),
transparency, and the shape of the stone. The stone should not be heavy
below the girdle; it should have a sufficiently sharp girdle to permit
the prongs to hold it firmly; it should have a symmetrical outline;
and its cabochon surface should not be too high nor too sharply domed.
The eye should follow the long diameter of the stone.

Asterism

Stones cut in cabochon that reflect light in the form of intersecting
bands of light are known as ASTERIAS. The words ASTERIA, ASTERIATED
AND ASTERISM are derived from the Greek word ASTER, meaning STAR. The
star may be four, six or even twelve rayed. Three, five and ten-rayed
stars are also known, but these are "freak" gems in which one ray of
the star is missing due to the absence of essential inclusions in one
area of the stone. Although stars are most commonly seen in ruby and
sapphire, other gems that occasionally display asterism are almandite
garnet, spinel, beryl, rose quartz and chrysoberyl.

In general, asterism is caused by reflection of light from minute,
oriented needlelike crystals. When the stone is cut en cabochon, the
reflected light is visible as bright intersecting lines. In star stones
of the corundum species, which crystallize in the hexagonal system,
numerous tiny elongated crystals are distributed in planes perpendicular
to the principal vertical crystal axis and oriented in directions parallel
to the faces of the hexagonal (six-sided) prism (Figure 3). The star-producing
inclusions in synthetic star ruby and sapphire are arranged in the same
pattern as in the natural.

It will be noted in Figure 3 that these minute crystals are oriented
in three directions and intersect at 60°,
through reflection, they produce three chatoyant bands at right angles
to the inclusions, thus forming a six-rayed star. When observing a cross
section of a crystal of this nature, one can see a surface sheen, which
is the result of light being reflected from these oriented crystals
within the stone. It is partly on this basis that one can determine
whether a crystal can be fashioned into a star stone. This surface sheen
can be resolved into a star by the use of the cabochon cut. The girdle
must be placed exactly parallel to the plane in which the needles lie,
in order for the star to be centered at the apex of the cabochon.

The minute needlelike inclusions that are essential to fine stars
are called SILK, because of the silky sheen they impart to the stone.
Needles coarse enough to be visible individually under 10x or 30x are
too large for sharp star production. Just as the large fibers in most
specimens of tiger's-eye produce only a wide chatoyant band, coarse
needles in any other gem mineral refection of the star in first-grade
Linde synthetic star sapphires and rubies is the result of the even
distribution and tiny size of the needles, or silk. Since synthetic
stars contain needlelike inclusions that are only clearly visible under
100x or higher magnification, the presence of coarse needles may still
be considered proof of genuine origin in corundum.

Aventurescence (pronounced ah-ven-chur-ESS-ence).

Aventurescence is the name applied to display of bright or strongly
colored reflections from a translucent mineral. It is seen in sunstone
( a variety of oligoclase feldspar) and in aventurine quartz (a spangled
green variety of massive quartz).

In feldspar it is caused by reddish, metallic reflections from minute,
flaky inclusions of a variety of hematite arranged parallel to planes
of repeated twinning. Fine sunstone resembles moonstone, but with a
golden instead of a bluish reflection. In quartz it consists of fairly
vivid reflections from green inclusions in a white background, or shiny
reflections from included flakes of mica in a green background.